Headgear assemblies are commonly used in orthodontic treatment to apply forces to a patient's teeth to accomplish specific types of tooth movements. Such headgear assemblies typically include an inner bow, an outer bow, and some type of a neck/head strap assembly. More particularly, the two ends of the inner bow are each typically inserted into a buccal tube attached to one of the upper first molars. Moreover, the outer bow is connected to the inner bow and extends at least partially about both sides of the patient's face such that the ends of the outer bow may be engaged by the neck/head strap assembly. The neck/head strap assembly is generally formed at least in part from an elastic material or other energy-storing mechanism so as to be capable of stretching and thus applying the desired treatment forces to each of the ends of the outer bow. These treatment forces are then typically transmitted to the upper first molars and any teeth interconnected therewith (e.g., via an archwire and/or other appropriate connectors).
The manner in which the above-described treatment forces are applied to the patient's teeth utilizing a headgear assembly may be varied to achieve the particular desired orthodontic treatment objectives. For instance, the force vectors which are utilized will affect the type of tooth movement achieved. A substantially purely distal or rearward force may be applied to the patient's teeth (i.e., a distal force vector) by positioning a neck strap about the back of the patient's neck and substantially within the occlusal plane for engaging the outer bow to achieve substantially only a distal tooth movement. In the event that a substantially symmetrical outer bow is utilized, substantially equal forces will be applied to each of the upper first molars. Although this substantially symmetrical distal pull may satisfy the treatment objectives in a number of patient cases, quite often various combinations of tooth intrusion/extrusion, as well unilateral or bilateral bodily movements of single or groups of teeth are required for an individual patient's treatment objectives.
In order to provide for the above-described multiple force delivery capabilities, headgear assemblies may incorporate a head strap assembly (e.g., a head cap), such as is generally illustrated in U.S. Pat. No. 3,186,089 to Asher, issued Jun. 1, 1965. With this type of a device, both distally and vertically directed force components can be effectively applied to the patient's teeth. By varying the location or angle at which the vertical force component is coupled relative to the occlusal plane, the above-described different types of tooth movements may be achieved. For instance, by applying the vertical force component mesially (i.e., forward) of the upper first molars which are again directly engaged by the inner bow, the upper first molars may be tipped distally (a distal root tip) while being bodily moved distally. Similarly, if the vertical force component is applied distally (i.e., rearward) of the upper first molars, the upper first molars may be tipped mesially (a mesial root tip) while being bodily moved distally.
In addition to varying the vectors of the treatment force to achieve a particular combination of simultaneous tooth movements, variations in force magnitudes applied to the upper first molars may also be utilized to achieve a desired result in orthodontic treatment planning. For instance, in some instances the forces which are applied to the upper first molars and teeth interconnected therewith are substantially equal, typically by utilizing a substantially symmetrical outer bow. However, there are also instances where it is desirable to apply a greater force on one side of the patient's mouth than on the other side such that one of the upper first molars and its interconnected teeth experience greater forces than the other upper first molar/interconnected teeth to achieve greater tooth movement. This technique is typically accomplished by utilizing an outer bow having different length arms (e.g., the arm being defined as the distance from where the outer bow is connected to the inner bow to the end of the outer bow), and/or by having one of the arms of the outer bow "bowed" to a greater degree than the other arm, and/or by various combinations thereof.
One of the problems generally associated with headgear assemblies in achieving a given treatment effect is that they are potentially hazardous to the patient from a health standpoint. More particularly, upon the application of external, non-orthodontic treatment forces to the headgear assembly, typically by a third party grabbing the patient's outer bow and exerting a force thereon, the potential exists for the inner and/or outer bow to undesirably impact the patient upon release of the external force if the treatment forces continue to be applied to the outer bow. This of course creates a risk for significant injury to the patient. For instance, if the outer bow is pulled out far enough so that the ends of the inner bow disengage the buccal tubes, the subsequent "snapback" of both the inner and outer bows upon termination of the application of the external force (e.g., by a disengagement of the outer bow by the third party), which is caused by the continued engagement of the outer bow by the neck/head strap assembly, is likely to cause damage to the soft tissue of the mouth by the impacting of the inner bow thereon. If the outer bow is pulled so that the ends of the inner bow actually extend outside the mouth prior to the termination of the application of the external force, injury to the patient's face or eyes may result due to the subsequent and above-described snapback and any resulting impingement of the inner and/or outer bow with the patient. As can be appreciated, neither of these scenarios is desirable in maintaining the safety of the patient at an acceptable level during orthodontic treatment. Therefore, headgear assemblies have incorporated some type of release mechanism to disconnect the assembly prior to the time at which an increased health risk to the patient exists due to these types of circumstances.
There are a variety of headgear assemblies available which utilize various type of mechanical releases. Generally, these mechanical releases are relatively complex from both a structural and thus manufacturing standpoint. Consequently, manufacturing costs may be undesirably increased. Moreover, these releases are subject to material fatigue (e.g., wear) due to the relatively high treatment forces involved and based upon the manner in which such releases function (e.g., utilizing a compression of members to pass through a opening to provide the disconnection), as well as contamination (e.g., collection of dirt/debris within the release). In addition, some of these releases are subject to patient manipulation. In any case, the point at which release occurs may be undesirably affected (e.g., it may increase or decrease via fatigue, contamination, and/or patient manipulation). U.S. Pat. No. 4,115,921 to Armstrong, issued Sep. 26, 1978; U.S. Pat. No. 4,238,188 to Armstrong, issued Dec. 9, 1980; U.S. Pat. No. 4,368,039 to Armstrong, issued Jan. 11, 1983; U.S. Pat. No. 4,155,161 to Armstrong, issued May 22, 1979; U.S. Pat. No. 4,215,983 to Frazier, issued Aug. 5, 1980; U.S. Pat. No. 4,402,669 to Frazier, issued Sep. 6, 1983; U.S. Pat. No. 4,226,589 to Klien, issued Oct. 7, 1980; U.S. Pat. No. 4,416,625 to Armstrong, issued Nov. 22, 1983; and U.S. Pat. No. 4,872,836 to Grove, issued Oct. 10, 1989, are generally representative of these types of mechanical release mechanisms.
Another type of a "release" mechanism for a headgear assembly is disclosed by U.S. Pat. No. 4,764,110 to Dougherty, issued Aug. 16, 1988. Dougherty discloses a headgear assembly in which the inner bow is detachably connected to displaced molars within a patient's mouth by a combination of buccal tubes and attractively engaged magnets. More particularly, a magnet is positioned on each side of the inner bow and on the buccal tubes as well such that when each buccal tube receives an end of the inner bow, the magnets come into contact. A magnet is also positioned on each of the inner and outer bows to attractively intercouple the same. These particular magnets may be positioned within a box-shaped retainer so as to confine the relative motion of such magnets, as well as the inner and outer bows, to lineal motion. An elastic band traverses the back of the patient's head and engages the ends of the outer bow to provide the treatment force thereto. By utilizing this configuration, the disclosure of Dougherty indicates that the separation of the inner and outer bows can be positively assured upon application of a predetermined force.
Based upon the foregoing and the illustrated configuration of Dougherty, it can be appreciated that none of the magnetic interconnections suggested by Dougherty affect the application of treatment forces to the outer bow by the elastic band. More particularly, regardless of whether the inner bow/buccal tube magnetic connection or the inner bow/outer bow magnetic connection releases at the described time, forces continue to be applied to the outer bow by the elastic band such that the potential for injury to the patient still exists due to the above-described snapback. Moreover, the types of treatment forces applied to the outer bow may have an effect on the point at which the described release occurs in the Dougherty configuration. For instance, the utilization of a treatment force having both distal and substantial vertical components may alter the release point by introducing binding (if the box-shaped retainer is incorporated which again limits relative motion to a line as noted) and/or by subjecting the magnetic interconnection between the inner and outer bows to compressive, torsion, and/or shear-type forces/stresses. Similar effects may be evident if "different" forces are applied to the ends of the outer bow as described above (e.g., by having the outer bow arms be of different lengths, being bowed to differing degrees, and/or a combination thereof).